Jointed or segmented bars can be utilized in a variety of circumstances in industry. It is desirable where jointing is undertaken to provide an arrangement which ensures full mating interengagement and is self-sustaining in interlocked relation, with registration, attachability and detachability of the respective sections readily accomplished and with the strength of the joint so established fully dependable within assigned limits of loading.
More particularly, the chain saw industry has adopted a jointed or segmented saw bar structure for a certain range of professional sizes and for certain range of domestic user sizes which segmented bar includes a main saw bar section and a replaceable rounded tip or nose section the nose section the nose section carrying a sprocket rotatably mounted therewithin on suitable bearings and being adapted to support and guide an endless saw chain as it is driven around the outermost curvate saw bar tip.
The adoption of the segmented saw bar has materially extended the useful life of the overall saw bar structure, in preserving the main saw bar section by allowing for repair or replacement of the rounded nose section which is normally subjected to greater loading abrasion and abuse.
Moreover, with the segmented saw bar the operator of the chain saw can readily detach and replace a damaged nose section in the field without any sophisticated tool or equipment.
A number of segmented or composite saw bar structures have been disclosed in prior published patents. As well several such structures have been manufactured and sold, some of which are reflected by those patents.
The following United States and Canadian patents illustrate a number of alternative proposals which will serve as background to those structures embodying the invention to be described and illustrated herein namely: U.S. Pat. Nos. 2,838,833, 2,888,964, 2,962,061, 3,124,177, 3,762,047 and 3,949,475. Canadian Pat. Nos. 506,987, 607,857, 696,847, 737,679 and 1,026,651.
The segmented saw or guide bar of the type under consideration can be derived from an elongated relatively heavy suitable steel bar or plate which is provided with a peripheral groove formation by a milling operation and subsequent heat treatment to provide a useable hardness.
It is common practice in the case of heavier saw bars which do not employ any sprocket or nose wheel to increase wear resistant of certain edges, particularly the edges of the rounded tip or nose section by applying a substance identified by the trade mark "Stellite" which increases resistance to wear and fatigue and consequent failure inflicted by frictional forces applied to such region particularly when the nose section is used for boring or plunge cutting.
The typical groove formation of a saw bar includes uniformly spaced apart rail formations arranged peripherally and outermost of the longitudinal and curvate edges of the tip or nose section, the dimensions of such groove formation being selected such that the bottom of the groove lies below the feet or tangs of centrally located drive links of the saw chain loop to be mounted upon the saw bar and so that the flanking rail formations support the tie straps or side links of such saw chain as it is driven in its endless path around the saw bar periphery.
Saw bars can also be fabricated from two-ply or three-ply laminates. The more common three-ply laminate is derived from three suitable steel bars or plates, the outer flanking plates typically having like dimensions and configuration and with the centrally located or sandwiched plate having a configuration corresponding to the flanking plates but of reduced dimensions so that when assembled and laminated the composite bar presents the aforementioned peripherally extending pair of rail formations outermost.
The rail formations of laminated bars particularly those portions bounding the tip or nose section being subjected to the same excessive wear and abuse likewise were usually protected by applying "Stellite" or other suitable material.
The introduction of direct drive chain saws lead to greatly increased wear and deterioration of saw bars. Solutions proposed included the adoption of the roller nose and the sprocket nose.
By modifying saw bars whether solid or laminated to provide a rotatable nose wheel or sprocket mounted upon bearings adjacent the nose end of such saw bar the saw chain could be carried around the nose out of close contact with the rail formation surfaces and so decrease frictional contact and wear.
An early example of a nose section carrying a sprocket is illustrated and described in U.S. Pat. No. 2,316,997.
Saw bar structures have been reduced in size, and the operating charcteristics of roller nose and sprocket nose bar have improved. However, the introduction of smaller chain did not lead to the adoption of smaller saw bar structures; rather it was the advent of smaller and lighter power heads that dictated the bar size and consequential chain pitch reduction. Yet the size reductions did not improve the operating characteristics of the roller and sprocket nose. These improvements were made by adjustments in design and processes brought about by failure analyses.
Notwithstanding such improved performance and increased life expectancy it was still desirable if not necessary to provide a segmented saw bar structure wherein the nose sections or assemblies could be readily replaced if damaged leading to the development of better jointing structures, for example, that revealed by U.S. Pat. No. 3,762,047 originating with the inventor named herein.
The segmented saw bar disclosed and claimed in U.S. Pat. No. 3,762,047 utilizes a main saw bar section presenting a bifurcated abutting end edge formation to the nose section and is adapted to receive mating spaced projections carried by the nose section the latter being provided with suitable reinforcing spacers therebetween to achieve a precise interfitting relationship when fully registered.
The bifurcations of the main saw bar section of U.S. Pat. No. 3,762,047 when subjected to requisite heat treatment or flame hardening tend to undergo dimensional changes in that region which afterwards must be corrected by grinding.
Moreover, with such arrangement it has been observed when the nose section is used for prying or is twisted the forces in shear applied to the bifurcations tended to cause early fatigue and failure.
Further, as will be observed the segmented saw bar of U.S. Pat. No. 3,762,047 includes abutting end edge formations which extend transversely of the longitudinal axis thereof and particularly in the region of abutment of the composite rail formations end edges lie in a common transverse plane.
An endless loop of saw chains on being driven against such rail discontinuity tends to have a standing wave imparted to it generating reactive forces which give rise to severe wear patterns in the bar periphery thereby shortening the useful life of such bar structure.